1. Field of the Invention
The invention relates to a seal assembly of the type employed for sealing an annulus defined between two telescopingly concentric conduits in a subterranean well, and particularly to an improved seal backup element for preventing extrusion of the sealing elements and method of fabricating same.
2. History of the Prior Art
Radially expandible annulus seals are employed in practically every subterranean well. Normally, the purpose of the seal is to seal the annulus defined between the bore of the well casing and the exterior of a tubing string extending down to the production formations. Such annulus sealing assemblies are an important part of packers, hangers, bridge plugs and the like. In many wells, the environment in which the annulus sealing assembly has to operate is not extreme, permitting the sealing elements to be formed from an elastomeric material. In other wells, and particularly steam injection wells wherein steam is transmitted into the well by one or more tubing strings to lower the viscosity of the hydrocarbons contained in the production formations, the environmental conditions can be quite extreme, involving temperatures in excess of 400.degree. F. and high pressures. Sealing assemblies for this type of well have generally utilized non-elastomeric materials such as sealing elements formed primarily from graphite which, of course, is not elastically deformable.
Regardless of the environmental conditions to which the sealing assembly is exposed, a common problem has been encountered in that the seal assembly is generally energized through the application of a substantial axial compressive force on the sealing elements, thus forcing such sealing elements to expand radially outwardly into sealing engagement with the wall of the annulus in which they are disposed. Whether the sealing elements are formed from elastomeric material or a nonresilient deformable material, such as a graphite composition, the sealing elements are subject to extrusion axially around the backup elements by which the compressive force is applied to a single sealing element or an axial stack of sealing elements. Such extrusion is highly undesirable in that it reduces the physical volume of the sealing elements and thus reduces the compressive forces acting on such sealing elements. Hence, the energizing mechanism for such sealing assemblies must be capable of maintaining a substantially constant compressive force on the sealing elements, irrespective of the fact that the volume of the sealing elements may be continually shrinking due to extrusion.
It is impractical to use a close fitting solid metallic backup element to minimize the extrusion of the material of the sealing element. The most popular form of seal backup element in recent years has been an annular mass of wire mesh which is originally knitted as a tube or stocking and then is dieformed into the desired annular configuration, which generally has a parallelogram cross section. The knitted wire mesh annular sealing elements are dieformed to a maximum dimension permitting them to be freely inserted within the annulus to be sealed. The subsequent application of an axial compressive force to such wire mesh backup elements results in not only the radial expansion of the sealing elements, but a radially outward deformation of the annular wire mesh backup elements into snug engagement with the bore of the outer conduit defining the annulus. A substantial force, however, is required to effect such outward deformation of the wire mesh backup element and this requires that the energizing elements for the seal be fabricated of stronger and hence, heavier individual elements. Obviously, a reduction in the force required to expand the wire mesh backup elements into snug engagement with the bore of the outer conduit defining the annulus to be sealed would be highly desirable in any type of sealing assembly.